The genetics of mammalian provides a unique system to study signalling and gene regulation. Alpha-melanocyte stimulating hormone (d-MSH), Agouti and Agouti related protein (Agrp), are three molecules, which when acting on at least five different melanocortin receptors (Mclr-Mc5r) affect pigmentation, grooming, sexual behaviour, learning, memory, responses to pain, as well as the regulation of body weight. Pharmacological studies suggest that the molecular interaction between Agouti/Agrp and the Mclr/Mc4r is quite distinct from that between melanocortins and their receptors. To gain further insight into this question, this project aims to determine which portions of the Mclr and Mc4r receptors are required for antagonist binding and to determine if these portions of the receptors affect antagonist and agonist action in the same manner. The specific aims include: the identification of specific domain/s that affects antagonist binding/action by generating chimaeric Mclr and Mc4r receptors; for a domain or domains that confer specificity of Agouti or Agrp action, site directed mutagenesis will be carried out to identify particular residues responsible for specificity; mutant receptors will also be tested for their binding and cAMP response to an array of small molecule melanocortins, which imbue each receptor with a unique pharmacological profile; in specific cases, use will be made of an in vivo assay using animals that ubiquitously express Agouti or Agrp, and loss-of-function mutations for the Mclr or Mc4r receptors.